The invention relates to a modulator for converting a first electric signal modulated on a carrier wave of a first frequency into a second electric signal modulated on a carrier of a second frequency, the first frequency being lower than the second frequency, comprising
a first input terminal for receiving the first electric signal,
an output terminal for supplying the second electric signal, and
a comb filter whose input is coupled to the first input terminal and whose output is coupled to the output terminal, the input of the comb filter being coupled to a first input of a signal-combination unit and by way of a delay means to a second input of the signal-combination unit whose output is coupled to the output of the comb filter, which modulator further comprises a second input terminal for receiving a third electric signal having a frequency which at least substantially corresponds to the first frequency and a third input terminal for receiving a fourth electric signal having a frequency which at least substantially corresponds to the second frequency, which second and third input terminals are coupled to the respective inputs of a first mixing stage which has an output coupled to a first input of a second mixing stage having a second input coupled to the first input terminal and having an output coupled to the first input of the signal combination unit.
Such a modulator is employed in the well-known PAL-V2000 video recorders to convert the chrominacnce signal which is read from a magnetic record carrier and is modulated on a carrier wave having a (first) frequency of approximately 625 kHz into a chrominance signal which is modulated on a (second) frequency of approximately 4.43 MHz, so that it is suitable for reproduction in a television set. The second mixing stage is arranged in the connection between the first input terminal of the modulator and the input of the comb filter. An oscillator supplying a carrier frequency of approximately 4.43 MHz is connected to the third terminal. A signal having a frequency of approximately 625 kHz is applied to the second input terminal. This signal is derived from the horizontal synchronizing signal by way of a phaselocked loop and therefore exhibits the same frequency variations as the carrier wave of the chrominance signal applied to the first input terminal. The result is that an up-converted chrominance signal with a fixed 4.43 MHz carrier is available on the output terminal.
It will be appreciated that for video recorders in accordance with the VHS or the video-8 system and for video signals in accordance with other standards, such as NTSC, other values (may) apply for the various frequencies. The comb filter in the modulator is used for reducing cross-talk arising in the chrominance signal read from the record carrier under the influence of the adjacnet tracks on the record carrier. The delay means provides a delay of 128 .mu.s, which corresponds to twice the line period. The delay is provided by a glass delay line.
The glass delay line for delaying chrominance signals with a 4.43 MHz carrier by 128 .mu.s is comparatively bulky, requires accurate adjustment and is expensive. It is the object of the invention to provide a modulator which can be more compact, which requires less adjustments and which can be cheaper if combined with other functions, such as for example skew correction.